Digital communications systems are those in which the information or data communicated is represented by discrete symbols which may be directly transmitted or used to modulate a carrier signal. Synchronous digital transmission systems are those wherein the operation of the transmitter and receiver must be synchronized to one another to accurately recover the transmitted data.
A variety of architectures are used for synchronous digital communications systems. One such architecture is the so-called dual-duplex architecture. In a duplex system, the data is transmitted at some predetermined rate over a communications channel in two directions. Each communications channel includes transmitter and receiver circuitry along with a communications signal path therebetween. This communications signal path can take a variety of forms, such as wire, optical fibers, or air.
In a dual-duplex arrangement, the data at some predetermined rate is evenly divided for each direction of transmission over two communications channels so that the data rate in each channel is one-half of the predetermined rate. The problem with such an arrangement is that the propagation delay of each channel is generally different and this difference may vary with time. As a result, accurate recovery of the transmitted data is, in general, not possible unless this propagation delay difference is compensated for via some form of synchronization.
Until recently, the provisioning of local subscriber "loops", i.e., communication facilities connecting a customer's business or residential premises with a local central office in the public communications network for high-speed digital transmission, required an engineering of each loop to meet error rate objectives. This engineering involved the removal of bridge taps and the installation of specifically-spaced signal amplifiers or repeaters. In upcoming industry offerings for providing high-speed digital signals over local subscriber loops, the need for such engineering has been eliminated. However, to meet the necessary signal cross-talk requirements, a dual-duplex architecture has been found to be the preferred system architecture. As previously discussed, the use of such an architecture, in turn, requires that, for each direction of transmission, the transmission of data in each channel be synchronized to the other.
While a variety of digital signal synchronization techniques are known, each such arrangement possesses significant shortcomings. In one class of known arrangements, for example, framing bits are periodically transmitted and detected to maintain synchronization. The use of such bits, however, either reduces the data rate that would otherwise be available to the customer or increases the required channel bandwidth. In another class of known synchronization arrangements, one or more training sequences, each including at least one a priori known signal, is transmitted at predetermined times, e.g., system start-up or the like, and these sequences are used to measure the propagation delay. The problem with the use of training sequences is that the transmission of customer data must be interrupted each time a training sequence is transmitted, and infrequent training sequence transmission does not provide satisfactory results in certain system applications as there is no measurement of, or compensation for, propagation delay variations which arise in the time interval between successive training sequence transmissions.
Based on the foregoing, it would be extremely desirable if a synchronization arrangement could be devised for digital communications systems which would not affect the bandwidth or customer data rate and which could provide frequent propagation delay determinations without interrupting the transmission of customer data.